Notch Stress Concentration Research Articles

Concept of “chemical notch” to study stress-corrosion

Abstract In this article, a new partially immersed slow strain rate (SSR) and corrosion-fatigue (CF) test methods are described and discussed. Partial immersion of a specimen in 3.5 % NaCl solution creates an interface between the solution and air. We call this a “chemical notch” stress concentration factor. The outcome of this demonstrates that the SSR test condition on a smooth specimen will fail at the interface if the machined notch stress concentration factor is less than the “chemical notch” stress concentration factor. This “chemical notch” concept can be applied to correlate corrosion-fatigue S–N data for a specific material/environment system.

Research on fatigue curve fitting methods based on the notch stress approach

PurposeThis study investigates the fitting techniques for notch fatigue curves, seeking a more reliable method to predict the lifespan of welded structures.Design/methodology/approachBuilding on the fatigue test results of butt and cruciform joints, this research delves into the selection of fitting methods for the notch fatigue curve of welded joints. Both empirical formula and finite element methods (FEMs) were employed to assess the notch stress concentration factor at the toe and root of the two types of welded joints. Considering the mean stress correction and weld misalignment coefficients, the notch fatigue life curves were established using both direct and indirect methods.FindingsAn engineering example was employed to discern the differences between the direct and indirect approaches. The findings highlight the enhanced reliability of the indirect method for fitting the fatigue life curve.Originality/valueWhile the notch stress approach is extensively adopted due to its accurate prediction of component fatigue life, most scholars have overlooked the importance of its curve fitting methods. Existing literature scantily addresses the establishment of these curves. This paper offers a focused examination of fatigue curve fitting techniques, delivering valuable perspectives on method selection.

Corrosion Fatigue of Standard Duplex Stainless Steel X2CrNiMoN22-5-3 under Rotation Bending Load in Northern German Basin Environment

Standard duplex stainless steel X2CrNiMoN22-5-3 is resistant to both, corrosion and mechanical stress, but corrosion fatigue (CF) lowers the lifetime expectancy in a geothermal environment such as the Northern German Basin. Laboratory experiments used the in-situ electrolyte at 369 K in a specifically designed corrosion chamber applying rotation bending cyclic load to failure. CF behaviour was compared to pure push-pull load. Corrosion kinetics are independent of the load applied. Failure is initiated by pits resulting in mechanical degradation. Increased thickness of the passivating layer surrounding pits enhances degradation or delamination. Also, sharp notches located along the pit edge increase notch effects and stress concentration consequently leading to fast crack propagation and early failure.

Effects of notch-load-defect interactions on the local stress-strain fields and strain hardening of additively manufactured 18Ni300 steel

This study investigates the influence of geometrical notches on the local (true) stress-strain curves, deformations, and strain hardening behavior of maraging tool steel 18Ni300 processed via the laser powder-bed fusion method as an additive manufacturing approach. For this purpose, five types of specimens with different notch designs were manufactured; these samples were considered to study the effects of the notch stress concentration factor and the notch position on the material's mechanical response against the applied external load. Accordingly, using the digital image correlation technique, true stress-logarithmic strain curves were plotted and compared for various points in the vicinities of the notches while the specimens were subjected to quasi-static tensile loads. Further, the strain (work) hardening behavior of the material at each point was then evaluated and compared with other points by plotting their strain hardening diagrams from the first derivative of the stress-strain curves. The results showed that the strain hardening of the samples increased with the stress concentration factor (notch sharpness) while its ductility decreased accordingly. Furthermore, notch location and shape also showed determining roles in defining the material behavior. Ultimately, higher stress concentrations, internal positioning, and less gradual changes in geometric features (C-shaped notches compared to V-shaped ones) can result in higher defect sensitivity, more decrease in ductility, and more likely catastrophic failures in metals processed by additive manufacturing.

Fatigue damage stiffness degradation modeling of right circular flexure hinges

Flexure hinges are susceptible to fatigue damage under cyclic loading, resulting in performance degradation. This paper investigates the stiffness degradation of the right circular flexure hinges (RCFHs) under cyclic loading. Fatigue damage experiments are conducted to obtain the stiffness degradation curves, which can be divided into several stages by feature points. A relationship between feature lives and alternating stress amplitudes is established. A fatigue damage stiffness degradation piecewise curve model for RCFHs is proposed. The effect of notch stress concentration on fatigue damage is analyzed. Fatigue damage experiments under non-zero mean stress are conducted, and an equivalent fatigue stress equation is obtained. Finally, a generalized fatigue damage stiffness degradation model for RCFHs is developed, which establishes a relationship between residual stiffness and cycle number. On this basis, a fatigue damage performance modeling method for flexure hinge mechanisms is proposed. The fatigue damage performance of a compliant bridge mechanism was modeled and tested. The experimental results of input stiffness degradation are generally in agreement with the predicted results, which verify the validity of the method.

Effect of carburizing process on bending fatigue performance of notched parts of 18CrNiMo7-6 alloy steel

Since stress concentration is easily formed at the notch, improving the fatigue performance of notched parts is critical. The carburizing process is widely used to manufacture notched parts. In this research, the effects of carburizing process and notch stress concentration on fatigue performance were analyzed by bending fatigue tests. In addition, the microstructure and hardness evolution processes of the carburized notched 18CrNiMo7-6 specimens were observed quasi-in situ. The results showed that macro-plasticity has little effect on the bending fatigue performance of notched samples and that the improvement in fatigue performance is primarily attributable to the high hardness and residual compressive stress introduced by carburizing. The fatigue notch sensitivity is therefore reduced after carburizing. Compared to the uncarburized specimens, the fatigue limit of the carburized specimens is increased by up to 128%. With the increase of the effective hardening layer depth, the fatigue limit increases first and then decreases. Moreover, the brittle phase transformation product (twinning martensite) of metastable retained austenite is easy to produce stress concentration. Conversely, the 'film-like' retained austenite with an extremely high stability can effectively avoid the phase transformation products.

Plastic stress concentration effects in fatigue strength

Elastoplastic (EP) stress gradient factors (SGF) ahead of notch tips are used to evaluate the actual notch effects in fatigue strength due to their tolerance to non-propagating short cracks, which depends on the notch stress concentration factor, on the stress gradient ahead it, and on the material. Considering the significant role of local plasticity in the growth behavior of short cracks within the notch plastic zone, a sound mechanical methodology is proposed to account for the effect of EP stress and strain fields in the actual fatigue stress concentration factor value, which includes its tolerance to short cracks. To validate the proposed methodology, life predictions based on numerical EP SGF are compared to experimental SN data of notched specimens made of different materials tested under uniaxial loads collected from the literature.

Fatigue performance of the slit end area of slotted CHS tube-to-gusset plate connection

Slotted circular hollow section (CHS) tube-to-gusset plate connections are frequently used in structures such as bridges, towers, etc., However the fatigue strength of this type of connections needs to be characterized. In the present study, the fatigue performance of the slit end area of this connection is investigated. Four sets of large-scale fatigue tests on the connection with coped hole at the slit end are carried out. Additionally, fatigue tests on CHS connections, available in the literature, are re-analyzed by means of effective notch stress (ENS) approach. Subsequently, parametric studies are carried out to analyze the influence of different geometrical and welding strategies on the notch stress concentration factor. The shortcomings of the ENS approach applied to this type connection are pointed out, proposing possible alternative solutions. Lastly, the optimal design for improving the fatigue strength of this slotted CHS tube-to-gusset plate connection is recommended.

Digital image correlation for accurate strain measurement on sharp notched specimens

Notch stress concentration and the notch effects in general, play a significant role in the field of structural mechanics. Also, the strain measurement in notches is still a challenge. In cases, when the notch geometry is not exactly defined e.g., weld joints etc., the measurement of strains in notches becomes even more important. Therefore, the present work focuses on the problem of strain measurement on notched specimens, especially on sharp notches (notches with small minimum radius) by using the digital image correlation method. Initial measurements were carried out on specimens with known stress concentration and the results were compared with the analytical and numerical solution. Multiple camera positions with respect to the specimen are compared and discussed. Minimum number of pixels per notch radius for accurate measurement was determined based on the measured data. This limitation is influenced by several effects. The presented work focuses deeper on the influence of postprocessing filter methods. Several possible solutions to overcome this limitation are presented. The obtained results and the knowledge gained by previous measurements were utilized by direct practical measurements, weld notch stress concentration measurement.

Fatigue Life Analysis of Aluminum Alloy Notched Specimens under Non-Gaussian Excitation based on Fatigue Damage Spectrum

In this study, a non-Gaussian excitation acceleration method is proposed, using aluminum alloy notched specimens as a research object and measured acceleration signal of a certain airborne bracket, during aircraft flight as input excitations, based on the fatigue damage spectrum (FDS) theory. The kurtosis and skewness of the input signal are calculated and the non-Gaussian characteristics and amplitude distribution are evaluated. Five task segments obey a non-Gaussian distribution, while one task segment obeys a Gaussian distribution. The fatigue damage spectrum calculation method of non-Gaussian excitation is derived. The appropriate FDS calculation method is selected for each task segment and the acceleration parameters are set to construct the acceleration power spectral density, which is equivalent to the pseudo-acceleration damage. A finite-element model is established, the notch stress concentration factor of the specimen is calculated, the large mass point method is used to simulate the shaking table excitation, and a random vibration analysis is carried out to calculate the accelerated fatigue life. The simulation results show that the relative error between the original cumulative damage and test original fatigue life is 15.7%. The shaking table test results show that the relative error of fatigue life before and after acceleration is less than 16.95%, and the relative error of test and simulation is 24.27%. The failure time of the specimen is accelerated from approximately 12 h to 1 h, the acceleration ratio reaches 12, and the average acceleration ideal factor is 1.125, which verifies the effectiveness of the acceleration method. It provides a reference for the compilation of the load spectrum and vibration endurance acceleration test of other airborne aircraft equipment.

Fatigue reliability analysis of metro bogie frame based on effective notch stress method

Fatigue cracks may occur at the weld toe or weld root in the fatigue control locations of the metro bogie frame, an effective fatigue assessment method is needed to analyze the bogie’s fatigue reliability thoroughly. In this paper, the fatigue property tests of typical welded joints were completed, combined with the effective notch stress concentration factor, the effective notch stress S-N curve was determined. The effects of joint dimensions were studied to get a safer S-N curve which could be applied to the fatigue reliability analysis. This paper provides a theoretical and practical basis for the fatigue assessment of the metro bogie frame under service conditions.

Parametric equations for notch stress concentration factors of rib—deck welds under bending loading

Parametric equations for notch stress concentration factors of rib—deck welds under bending loading

Notch (stress concentration) factor estimation of a cylinder under internal pressure using different approaches

Abstract Today, fluid transportation via pipes can be found in many sectors. Therefore, safe fluid transportation possesses critical importance. While working, transportation pipes are exposed to unwanted loads that culminate in stresses which cause deformation on the part geometry especially in sharp corners, holes or sudden cross-section change areas considered as notched. The notch effect parameter is considered in the mechanical design formulas. This study is interested in the notch factor that is estimated for a cylinder which undergoes an inner pressure. Some users can use false numerical values due to misreading or lack of attention. Because of this reason, graphs were converted to the numerical value by using computer software. In this study, Peterson’s chart was accepted as scientifically valid. Stress concentration factors were obtained by using four other approaches. These are regression, analytical, artificial neural network and finite element analysis. Among these models, high accuracy values were given by the artificial neural network model.

Characterisation of actual weld geometry and stress concentration of butt welds exhibiting local undercuts

Nowadays automated welding processes produce high-quality welds with smooth and regular global weld geometry. However, the local geometry of the weld exhibits a certain amount of variance in its topography due to dynamic effects of the welding process itself. The subsequent change in local stress concentrations affects the fatigue behaviour of welded structures. In literature, numerous equations for the estimation of a weld’s stress concentration based on characteristic parameters of the weld geometry are available.Three-dimensional scans of ultra high-strength steel butt welds exhibiting local undercuts are processed with digital image acquisition to derive weld geometry parameters, which facilitate a subsequent statistical analysis. Hereby, a novel approach to estimate the local weld toe radius is presented. Furthermore, the derived values are utilized to calculate the respective stress concentration at various locations of the weld using state-of-the-art notch-factor approximation equations. For undercut-free samples, a comparison of the investigated notch stress concentration equations and the results of numerical simulations of the actual weld geometry show good accordance for all models. However, at locations with a distinct undercut, the stress concentration is consequentially underestimated. Therefore, an improved approach extending the existing notch-factor equations is presented. It enables the consideration of the stress concentrating effect of a local undercut at the weld toe.

Fatigue fracture assessment of 10CrNi3MoV welded load‐carrying cruciform joints considering mismatch effect

AbstractFatigue experiments and numerical simulations based on the linear elastic fracture mechanics (LEFM) theory were conducted on the evenmatched (EM) and undermatched (UM) 10CrNi3MoV load‐carrying cruciform welded joints (LCWJs). The study firstly experimentally investigated the fatigue crack growth rate (FCGR) of base metal, EM, and UM weldments. The corresponding Paris parameters as essential input data are provided to assess the fatigue crack propagation behavior for weld toe and weld root failure caused by notch stress concentration variations. On the one hand, the stress intensity factors (SIFs) at weld toe and weld root were calculated considering the effects of LCWJ specimen geometries, initial crack types, and sizes. The comparisons between simulated results and standards analytical solutions were executed, which exhibit good accordance. It proved that the fatigue fracture simulation procedure based on LEFM is appropriate for the fatigue assessment of LCWJs. Eventually, it conducted the parametric analysis by predicted S–N curves, which included in the weld length, initial crack shape, initial crack size, penetration length, and materials fracture parameter, to explore some safety assessment reference lines for both failure modes of LCWJ.

Effective-notch-stress-based fatigue evaluation of rib-deck welds integrating the full-range S—N curve concept

Compared with the nominal and hot-spot stress, the effective notch stress could present more accurate stress state by involving the notch stress induced by weld bead. Inspired by the concept of full-range SN curve, a new notch stress-based fatigue evaluation framework is proposed in this study. The proposed fatigue evaluation framework reasonably considers the material types, welding residual stress, mean stress effect and notch stress concentration. Fatigue testing data of 60 rib-deck welded specimens are extracted to validate the feasibility of the proposed framework. Field monitoring in a real cable-stayed bridge was also carried out as a case study to present the application of the proposed framework. Results show that the proposed fatigue evaluation framework is feasible and reliable for the evaluation of fatigue testing results of rib-deck welded specimens. Instead of the Goodman method and SWT method, Walker equation is recommended to take into consideration the mean stress effect. In engineering application, the proposed fatigue evaluation framework provides more conservative prediction results compared with code recommended FAT225 SN curve, which is applicable to important components where fatigue cracking is not allowed.

Effects of grinding treatment on surface properties and deformation microstructure in alloy 304L

Different types of sandpapers were used to grind surface of 304L alloy, and the effects of grinding treatment on surface properties and deformation microstructure were studied. Three dimensional profilometer, Vickers hardness tester, X-ray diffraction (XRD), scanning electron microscopy (SEM), focused ion beam (FIB), and transmission electron microscopy (TEM) were used to characterize the microstructure and mechanical properties. The results showed that with the increase of grinding particle size the surface roughness and hardness increased, while the absolute value of residual compressive stress decreased, however the grinding particle size had no effect on the average tension strength because the groove did not lead to the effect of notch stress concentration. After grinded by coarse sandpaper (No. 60), the surface metal deformed in the way of martensite shear from γ-austenite to α′-martensite, which conformed to K-S model, and surface nano-crystal layers with a thickness of about 2.3 μm, 1.5 μm and 0.6 μm were found at the groove peak, slope and valley, respectively. By comparison, after grinded by fine sandpaper (No. 2000), the nano-crystallization did not occur, and a high-density dislocation layer with a thickness of more than 1 μm was observed at the groove peak, slope and valley. • Hardness increase and compressive stress decrease with the increase of roughness. • Roughness had no effect on tension strength and notch stress concentration. • Surface metal deforms in the way of γto nano-α′ after grinded by No. 60 sandpaper. • Thickness of nano-α′ layer at groove peak, slope and valley decreases gradually. • Surface metal deforms as dislocation slip after grinded by No. 2000 sandpaper.

Parametric Formula for Stress Concentration Factor of Fillet Weld Joints with Spline Bead Profile.

The existing parametric formulae to calculate the notch stress concentration factor of fillet welds often result in reduced accuracy due to an oversimplification of the real weld geometry. The present work proposes a parametric formula for the evaluation of the notch SCF based on the spline weld model that offers a better approximation of the real shape of the fillet weld. The spline model was adopted in FE analyses on T-shape joints and cruciform joints models, under different loading conditions, to propose a parametric formula for the calculation of the SCF by regression analysis. In addition, the precision of parametric formulae based on the line model was examined. The magnitude of the stress concentration was also analyzed by means of its probability distribution. The results show that the line model is not accurate enough to calculate the SCF of fillet weld if the weld profile is considered. The error of the SCF by the proposed parametric formulae is proven to be smaller than 5% according to the testing data system. The stress concentration of cruciform joints under tensile stress represents the worst case scenario if assessed by the confidence interval of 95% survival probability.

Multiaxial low cycle fatigue assessment of notch specimens for 10CrNi3MoV steel and its welds

Due to the complex multiaxial loading service conditions for welded structures, it is a fundamental step to accurately compute the fatigue damage under complex stress states. Multiaxial low cycle fatigue behaviors of notch specimens considering geometry variations and strength heterogeneity for 10CrNi3MoV high strength steel and weldments under proportional and non-proportional loading modes were investigated in our study. The experimental results show that the crack initiates from the notch tip. The characteristic states of normal stress, shear stress and energy are analyzed to express the notch gradient behaviors. Furthermore, a new simplified fatigue damage parameter is proposed to predict the multiaxial low cycle fatigue life of notch components. This damage parameter is combined with Smith-Watson-Topper (SWT) damage parameter and notch stress concentration factor in the proximity of the notch tip. Finally, the proposed model gives more satisfactory estimations than other notch energy damage models for low cycle multiaxial fatigue life.

Mechanical Properties of Remote-Laser Cut CFRP and Thermographic Laser-Process Monitoring

Remote-laser beam cutting is a productive technology without tool wear. Especially when cutting carbon fiber reinforced polymers (CFRP), it offers constant manufacturing quality. Since it is a thermal process, a heat-affected zone (HAZ) is formed at the edge of the cut. Based on quasi-static and cyclic mechanical tests on open-hole specimens, the influence of the process on the mechanical properties of CFRP is shown. The quasi-static tests are in good correlation with results from other researchers by indicating an increase in the maximum tensile stress of the test specimens, cut by remote-laser. The reason is the rearrangement of the shear stresses and a reduction of the notch stress concentration. However, the results of the present study show that excessive expansion of the HAZ leads to a reduction in the maximum tensile stress compared to milled test specimens. Under cyclic load conditions, remote-laser beam cutting does not lead to a more pronounced degradation than milling. The mechanical properties of the notched test pieces are sensitive to the expansion of the HAZ. For the production of components it is therefore necessary that the remote-laser beam cutting is carried out under controlled and documentable conditions. For this purpose, process thermography was tested as a tool for quality assurance. The results show that the technology is basically suitable for this task.